Fail-safe detector

ABSTRACT

A fail-safe position detector in which a pulsed light source induces corresponding electrical pulses in a light responsive receiver circuit when the components are in a desired relative position. The pulses generated in the receiver circuit are effective to operate a translating device through an isolation transformer. Threshold devices at critical junctures in the circuit eliminate marginal operation. The components are selected so that failure of one component or a combination of components will not cause false triggering of the translating device.

FAIL-SAFE DE'iECTOR Inventors: William E. Zitelli; Charles L.

Winkler, both of Pittsburgh, Pa.

Primary Examiner-Bernard A. Gilheany Assistant Examiner-W. E. Duncanson,Jr. Attorney-A. T. Stratton, D. R. Lackey and R. V.

[73] Assignee: Westinghouse Electric Corporation, Westerhoff Pittsburgh,Pa. 22 Filed: May 19, 1971 [57] ABSTRACT A fail-safe position detectorin which a pulsed light [21] Appl 4:726 source induces correspondingelectrical pulses in a light responsive receiver circuit when thecomponents [52] US. Cl. 187/29 R, 318/480 are in a esi ed elativeposition. The pulses generated [51] Int. Cl B66b 1/36 in the receiver uiar tiv t operate a trans- [58] Field of Search 187/29; 318/480; i gdevice hr ugh n i olation ransformer. Thresh- 307/1 17 old devices atcritical junctures in the circuit eliminate marginal operation. Thecomponents are selected so [56] R f r Cit d that failure of onecomponent or a combination of UNITED STATES PATENTS components will notcause false triggering of the trans- 3,509,359 4/1970 Embling 307/117latmg devlce' 3,138,357 6/1964 Whitwell et a1 318/480 X 12 Claims, 5Drawing Figures FUNCTION L 2 GENERATOR 5 LINE 5 PHOTO TRANSISTOR 7DRIVER r 12 K '4 4 THRESHOLD HIGH PASS THRESHOLD AC TRANSLATING #1 8 9FILTER #2 COUPLER DEVICE PAIENIED JUL 3 I915 3. 743. 056

2ND LANDING 22 26 22 IST LANDING B FIG. 3 5

FAIL-SAFE DETECTOR BACKGROUND OF THE INVENTION 1. Field of the InventionThe invention relates to detector devices for detecting the relativepositions of objects and is illustrated as applied to detecting thelocation of an elevator car relative to the landings in a hoistway.

2. The Prior Art Various schemes and devices have been utilized-todetect the position of an elevator car relative to the landings in thehoistway. The simplest such device is a cam operated mechanical switch.Such devices are subject to mechanical wear and misadjustment.

Another widely used device is the inductor relay. The inductor relay isan electromechanical relay in which the armature is not displaced uponenergization of the coil until a magnetic circuit is completed by aseparate magnetically permeable member. Uusually the coil is mounted onthe car and a vane of the magnetically permeable material is mounted inthe hoistway at a point where it will complete the magnetic circuit ofthe relay as the car passes in close proximity in the vane. Such deviceseliminate the problem of mechanical wear, however, theylack the abilityof precisely locating the position of the car.

Inductor systems which are capable of generating a continuously variablesignal indicative of the displacement of the car from a landing are alsoin use for bringing the car rapidly and accurately to a landing. Such asystem is disclosed in US. Pat. No. 2,874,806 and an improvement isshown in US. Pat. No. 3,207,265. Electrostatic devices have also beenused to a limited extent for controlling the operation of the car as afunction of the relative position of the car.

In addition, visible light beams have been used in conjunction withphotoresistors to accurately establish the position of elevator carsrelative to the landings. In US. Pat. No. 3,138,223, a beam of lightgenerated by an incandescent bulb energizes a photoresistor through aslot in a vane mounted on the wall of the hoistway when the car is levelwith the landing. The device is also used to indicate when the car iswithin a predetermined distance of the landing during slowdown so thatdoor opening may be initiated. Incandescent lamps and photoresistors arealso in wide spread use for determining when passengers are in the pathof automatically operated elevator doors to prevent the doors fromclosing on passengers and for purpose of monitoring passenger transfers.

The photoelectric detecting devices utilized to date are responsive to acontinuous light signal and are therefore subject to false triggering byrandom light sources. In addition, neither the incandescent lightsources nor the photoresistive devices can be operated at anysignificant pulse rate which would be useful in reducing the incident offalse triggering caused by random light sources.

SUMMARY OF THE INVENTION According to the invention, a pulsed source ofradiant energy, preferably visible light or infrared light, triggers aphotoresponsive device in a receiver circuit when two objects are in adesired relative position with respect to each other. Correspondingelectrical pulses generated in a receiver circuit operate a translatingdevice, which may take the form of an electromagnetic relay, through analternating current (hereinafter referred to as A.C.) coupling device.

Preferably, the pulsed source of radiant energy is in the form of alight emitting diode which may he pulsed at a frequency in the kilohertzrange. In the preferred embodiment of the invention, the A.C. couplingdevice is an isolation transformer which couples signals in thekilohertz range applied through the primary to the secondary. By pulsingthe radiant energy source and coupling the translating device to thereceiver through an A.C. coupling device tuned to the frequency of theradiant energy source, misoperation caused by stray radiant energy orfailure of a component in the receiver circuit in an activated conditionis minimized.

In order to further improve the reliability of the detector, a thresholddevice compares the amplitude of the electrical pulses generated by thephotoresponsive device to a reference signal and only passes pulses of apredetermined amplitude to the isolation transformer. In the embodimentof the invention disclosed in detail, the threshold device includes anoperational amplifier utilized as a comparator and a Zener diode havinga breakdown voltage selected such that pulses pass through the Zeneronly when they have a value with respect to the reference signal whichis determined by the predetermined breakdown voltage of the Zener diodeand the transfer function of the operational amplifier.

The operational amplifier is an uncompensated microcircuit operationalamplifier which has a natural frequency in the megahertz range so thatoscillations due to failure of the operational amplifier will not bepassed through the isolation transformer to the translating device. Ifthe translating device is to be located remotely from the receiver, aline driver and a line receiver may be utilized in conjunction with anA.C. coupled transmission line between the threshold device and theisolation transformer to reduce the possibility of noise interference.

For detecting a desired relative position between two objects, thetransmitter may be mounted on one object and the receiver on the otheror both the transmitter and receiver may be mounted on the same objectwith a control element on the other object to control the flow of pulsesfrom the transmitter to the receiver. The control element may eitherinterrupt or complete the flow of pulses between the transmitter and thereceiver. In the latter case, it may take the form of a reflectivesurface. In the embodiment of the invention described in detail, thecontrol device is in the form of a vane which projects from one objectperpendicular to the direction of the relative motion of the twoobjects. The pulses of radiant energy are focused into a beam whichintersects the line of sight of the receiver in the plane -of thereflective surface. To further increase the reliability of the device, anon-reflective surface is placed in the line of sight of the receiver ata point beyond the plane of the reflective surface to protect thereceiver from stray radiation. More than one transmitterreceivercombination can be used on separate channels to determine the directionof approach of the object to a desired position. The device may also beused to monitor passenger transfers between the elevator car and thelanding by locating the transmitter and receiver so that a passengerpassing through the doorway will interrupt the light pulses.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a block diagram of adetector device according to the invention; 1

FIG. 2 is a schematic diagram of an elevator system incorporating theinvention;

FIG. 3 is an enlarged sectional view of a portion of FIG. 2; and

FIG. 4 is a schematic circuit diagram of a detector according to theinvention; and

FIG. 5 is a plan view in section of another application of the inventionto an elevator system.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates in blockdiagram form the general organization of the invention. A functiongenerator 1 generates a repetitive electrical signal which drives alight-emitting diode 2. A light-emitting diode which emits light in thevisible spectrum can be utilized, however, a General Electric infraredlight emitting diode, SSL-5C, has been utilized in the preferredembodiment of the invention. The infrared light-emitting diode wasselected because its emissions can be reflected by a metallic surfacewhich is advantageous as will be seen below. It should be understoodthat any control device which can emit electromagnetic radiation havinga wavelength in the range of approximately 2,000 to 10,000A could beutilized. The light-emitting diode has the advantage of long life andcan be pulsed at frequencies in the megahertz range for the state of theart lightemitting diodes. Incandescent light sources on the other handhave limited life and cannot be pulsed at any significant pulse rate dueto residual incandes' cence.

A function generator which generates any repetitive electrical signalcould be utilized for the function generator 1, however, a square wavegenerator which generates a signal having a frequency of 4 kilohertz wasselected for the preferred embodiment. The lightemitting diode 2(hereinafter referred to as LED) therefore emits pulses of infraredradiation having aat a phototransistor 4 or may be reflected from areflective surface 5. As mentioned above, the infrared energy may beeasily reflected from a polished metallic surface.

The phototransistor 4 is operative to generate electrical pulses havinga frequency corresponding to that of the pulses of infrared energy 3triggering it. The phototransistor is capable of generating pulses inthe kilohertz range unlike the photoresistor which is commonly referredto as the photocell.

A first threshold device 6 passes along the electrical pulses generatedby the phototransistor only when they exceed a predetermined value. Thisadds to the reliability of the detector byrejecting electrical pulsesgenerated by stray or reflected infrared energy. Signals which exceedthe threshold of the threshold device 6 are amplified by the amplifier7. The output of the amplifier 7 is applied to the translating device 14through an A.C. coupler 13. In a preferred embodiment of the invention,the A.C. coupler 13 is in the form of an isolation transformer which isoperative only to couple signals in the kilohertz range. If thetranslating device and coupling device are located remotely from theamplifier 7, a transmission line 9 having a line driver 8,.a high passfilter 10, a second threshold device II and an additional amplifier 12are provided to assure the delivery of a reliable signal to the A.C.coupler 13.

FIG. 2 illustrates an elevator system incorporating the invention. Anelevator car 15 is suspended by a wire rope 16 in a hoistway 17. Thewire rope 16 passes over a drive sheave l8 and supports a counterweight19 in the conventional manner. The elevator car 15 is caused to move upand down in the hoistway 17 to serve three landings by a motor andcontrol system 19 which acts through the shaft 20 supporting a drivesheave 18.

The transmitter and receiver of the position detector are mounted in aunit 21 on the side of the car. The flow of the pulses of infraredenergy between the transmitter and receiver are controlled by vanes 22mounted at each landing. These vanes may be the vanes associated withthe inductor landing system described in U.S. Pat. No. 3,207,265mentioned above. Strips of metallic material 23, or other materialswhich reflect infrared rays, are fastened to the vanes 22 to provide areflective surface for the pulses of infrared energy. In the arrangementshown, pulses of infrared energy will be reflected by the strips 23 whenthe elevator car 15 is a predetermined distance above or below eachlanding. It is clear that various other arrangements of the reflectivestrips can be arranged to indicate any desired position of the car inthe hoistway such as when it is in exact registry with each landing. Theindication that the car is within a given distance above or below alanding can be used for various elevator control functions such as thepreopening of the door as the car approaches a landing and for levelingpurposes. By using pairs of transmitters and receivers and additionalstrips of reflective materials, a plurality of channels can be provided.The information provided by the various channels can be combined in anynumber of ways to perform desired logic functions. Suitable materialsfor the reflective strips 23 include chromium plated plastic or metal,polished copper etc. Gold and silver polyester tape sold under the tradename of Mylar have been found suitable for reflecting infraredradiation. Mylar tape has the advantage that it can be easily cut to thedesired dimensions and is provided with an adhesive backing for easyinstallation.

If the translating device is to be located in the control 19 located inthe penthouse, the transmission line 9 shown in the block diagram ofFIG. 1 can be routed to a junction box 24 on the car and then throughthe traveling cable 25 to the junction 26 in the hoistway. Thetransmission line 9 can then form part of the cable 27 which connectsthe junction box 26 with the control 19.

FIG. 3 shows an enlarged plan view of the section of the elevator systemshown in FIG. 2. The detector unit 21 fastened to the car 15 mounts thetransmitter 2 so that the beam of pulses formed by the lens or focusingdevice 28 intersects the metallic strip 23 on the vane 22 mounted on thewall of the hoistway 17 when the car is adjacent to the landing. Thebeam of pulses reflected from the metallic strip 23 are focused by thelens 29 on the photoresistor 4. A shield 30 projecting from the car 15in the line of sight of the receiver 4 protects the receiver from randomor reflected infrared radiation when the car is not adjacent thelanding. it can be appreciated that when the car is not adjacent a vanethe beam of pulses generated by the transmitter 2 will not be reflectedinto the line of sight of the phototransistor 4.

FIG. 4 is a schematic circuit diagram of a preferred embodiment of theinvention. A supply voltage of plus 125 volts DC. is applied betweenbuses 30 and 31 with the bus 30 being held at ground potential. TheZener diode ZDl cooperates with the resistor R4 to provide a 30 voltsupply for the squarewave generator portion of the circuit. Although anyconventional squarewave generator could be utilized, the squarewavegenerator utilized in the circuit of FIG. 4 comprises the programmableunijunction transistor PU and the conventional J-K flip-flop F-F. Theprogrammable unijunction transistor PU is a solid state device which hasa'characteristic that it will block the passage of current from itsanode A to its cathode K until its anode to cathode voltage exceeds, bya small predetermined amount, the voltage between its gate electrode Gand the cathode. When this occurs the device exhibits a negativeresistance characteristic much as an ordinary unijunction transistorexhibits. The voltage at which the device will conduct is determined bythe voltage applied between the gate and the cathode, hence the originof the term programmable.

As used in the circuit of FIG. 4, the voltage at which the programmableunijunction transistor PU will fire is determined by the voltage dividercomprising theresistors R2 and R3. The anode to cathode voltage of PU isdetermined by the voltage on a capacitor C1 as it is charged through theresistor R1. Until the charge on a capacitor C1 reaches thepredetermined value, the voltage determined by the voltage divider R2and R3 is applied to the input 32 of the flip-flop F-F. When the chargeon the capacitor reaches the predetermined firing voltage, theprogrammable unijunction transistor PU fires to rapidly discharge thecapacitor C1 through the current limiting resistor R26. Since the valueof the resistor R26 is very small, the input to the flip-flop F-F goesto essentially ground potential. When the capacitor C1 has discharged,the programmable unijunction transistor is reset and the input voltageto the flip-flop F-F is again the voltage determined by the resistors R2and R3 while the capacitor C1 recharges. It can be seen then that thepotential applied to the input 32 of the flip-flop F-F alternatesbetween essentially zero voltage and some predetermined positive value.

The flip-flop F-F is the conventional J-K type of flipflop in which asignal appears on the output 33 upon alternate applications of a signalto the input 32. In such flip-flops the state of the output signalremains constant despite the removal of the input signal until anotherinput signal is applied. Assume that the output 33 of F-F goes to somepositive value while the capacitor Cl is charging. When PU fires so thatthe input to flip-flop F-F goes to zero, the signal at the output 33remains equal to one. When the capacitor C1 has discharged and thesignal applied to the input 32 again goes to some positive value, theoutput signal at 33 goes to zero and remains at zero when PU againfires. This time when C1 has discharged and PU is again reset so that apositive signal is applied to the input 32, the output 33 returns to theoriginal positive value. Thus it can be seen that the output 33 of theflip-flop F-F alternates between zero volts and some positive value withthe frequency of the pulses being determined by the cycle time of theprogrammable unijunction transistor PU. The Zener diode ZD2 and theresistor R5 combine to sharpen the waveform of the signal developed at33.

The squarewave signal thus generated is amplified by the two stageemitter follower amplifier comprising NPN transistors Q1 and Q2 whichserve to control the light-emitting diode LED. When the output 33 is atzero so that transistors Q1 and Q2 are turned off, LED receives currentthrough resistors R10 and R9 and a diode D1 and emits light in theinfrared range. When a positive signal appears at the output 33 of RF,transistor O1 is turned on. The base current thus supplied to Q2saturates this transistor. in this state, the collector to emitterimpedance of Q2 drops to essentially zero thereby shorting LED andterminating its emission of infrared light. The diode D1 having aforward drop which exceeds the collector to emitter impedance of Q2 whenit is in saturation assures that LED is cut-off under thesecircumstances. It is seen then that the LED generates pulses of infraredradiation as a function of the squarewave signal appearing at the outputof the flip-flop 33.

The receiver circuit includes phototransistor PT and a threshold devicewhich includes the operational amplifier 0A operated as a comparator anda Zener diode resistor combination ZDS and R19. The Zener diode ZD4cooperates with resistor R11 to establish a supply voltage of 30 voltsfor the phototransistor PT and the operational amplifier OA. Zener diodeZD3 cooperates with resistors R11 and R12 to establish a referencevoltage which is applied to inputs A and B of the operational amplifierthrough resistors R13 and R14 respectively. The resistor R15 which isvery large compared with resistors R13 and 14 biases the potential atinput A to be slightly negative with respect to the potential at input Bof the operational amplifier. The incidence of infrared radiation uponthe phototransistor PT causes additional current to pass throughresistor R15 thereby making the potential of the input A of theoperational amplifier positive with respect to the input B.

- This results in a positive potential at the output of OA.

When this potential exceeds the Zener breakdown voltage of the Zenerdiode ZDS, base current is supplied to the transistor Q3 which turnsthis transistor on.

The transistor Q3 controls the line driver which includes the transistorQ4. With Q3 cut-off, Q4 of the line driver is turned on to supplycurrent through resistor R17, transmission line 9, resistor R21, Zenerdiode ZD6 and the base to emitter circuit of transistor Q5 'to chargethe capacitor C2 of the high pass filter comprising capacitor C2 andresistor R21. in this state the transistor Q5 can be turned on by thebase current applied through the resistors R22 and R23. The transistorQ4 is biased to the on condition by the resistor R18 so that the linedriver will supply current to maintain C2 in the charged state despitethe introduction of negative spikes on the line.

With the transistor Q5 turned on, transistor O6 is turned off so that nocurrent passes through the primary of the transformer T1. When thetransistor O3 is turned on by a voltage which exceeds the Zenerbreakdown voltage of the Zener diode ZDS, the base of transistor Q4 goesto essentially zero volts to turn Q4 off. The capacitor C2 will thendischarge through the circuit formed by resistor R17, diode D2, thecollector to emitter circuit of transistor Q3, bus 30, diode D3,

Zener diode ZD6 and resistor R21 as long as the charge on the capacitorexceeds the Zener breakdown voltage of ZD6. This causes the base oftransistor Q to go negative with respect to the emitter which turns offQ5. The diodes D2 and D3 complete the discharge path and protect theemitter to base circuit of the transistors Q4 and Q5 respectively bylimiting reverse bias to the forward drop of the diodes.

With transistor Q5 turned off, transistor O6 is turned on to complete acircuit for the energization of the primary of the transformer T1. Abuild-up of the magnetic field in the primary T1 induces current in thesecondary which is rectified by the full wave rectifier bridge circuitB1 which supplies direct current to the coil of the relay R. Thecapacitor C4 serves as a filter for the bridge circuit. When thetransistor O6 is again turned off by the turning on of the transistorQ5, the collapse of the field in the primary Tl again induces a pulse inthe secondary. Continued pulsing of the transformer T1 generatessufficient direct current to maintain the relay R in the energizedstate. The capacitor C3 is provided to protect the transistor Q6 fromspikes caused by the discontinuities of the current in the primary ofthe transformer T1.

' It can be seen then that the relay R will be energized only as long asthe photo transistor is subjected to pulses of infrared radiationemitted by the light emitting diode LED. The strength of the incidentinfrared radiation must be sufficient to drive the A input of OAsufficiently positive with respect to the B input so that the output of0A exceeds the Zener breakdown voltage of ZDS. The value of theelectrical signals generated by the phototransistor must exceed thereference voltage by an amount which is a function of the breakdownvoltage of the Zener diode and the transfer function of OA. If thevoltage output of 0A does not exceed the breakdown voltage of ZDS, Q3will not be turned on and no pulses will therefore be induced in theprimary of the transformer T1. As mentioned previously the line driverwhich includes transistor Q4 cooperates with the capacitor C2 to assurethat spikes on the line will not erroneously generate pulses in theprimary of T1. The Zener diode ZD6 assures that the pulses which will bepassed to the translating device must be above a certain threshold toeliminate false triggering by stray signals induced in the transmissionline. The A.C. coupling provided by the transformer T1 precludes falseenergization of the relay R due to failure of a component such as O6 inthe activated condition. Furthermore, T1 is an isolation transformerwhich is constructed so that only signals in the kilohertz range appliedto the primary will induce current in the secondary. This furtherimproves the reliability of the system by precluding false triggering bystray signals in other frequency ranges.

The operational amplifier 0A is a high gain uncompensated operationalamplifier which is operated in an open loop configuration to precludefalse triggering of the relay R should the operational amplifier fail inthe oscillating state. As a further precaution, 0A is a microcircuitoperational amplifier which has a natural frequency above one megahertzso that failure of the operational amplifier in this state will not beeffective to induce current in the secondary of the transformer T1. 6

FIG. 4 is so designed that failure ofa single component or a combinationof components will not falsely energize the relay R. This is a highlydesirable feature in a passenger transportation system where fail-safeoperation has'high priority.

FIG. 5 illustrates how the detector according to the invention can beutilized in place of the present light detectors used to control theautomatic operation of elevator car doors 32. In this arrangement, thetransmitter may be placed on one side of the entranceway 33 so as toproject pulses of infrared energy across the entranceway to the receiver4 mounted on the other side of the entranceway. One of the units couldeven be mounted to the door as is shown in FIG. 5. In thisconfiguration, the relay R will be energized as long as no one is in thedoorway, but will be dropped out when the beam of pulses of infraredenergy is interrupted by a passenger or an object in the doorway.

It is apparent from the above discussion that the invention could beutilized in many applications where it is desirable to have a reliablefail-safe detector. The light emitting diodes have an advantage over theuse of incandescent lamps not only in the fact that they can be pulsedat high frequencies thereby reducing the possibility of falseindications due to stray radiation, but also in their exceptionally longlife as compared with incandescent light sources.

We claim as our invention:

1. A fail-safe position detector for determining when two objects are ina desired relative position with respect to each other comprising:

electrical energy means,

a transmitter connected to said electrical energy means,vsaidtransmitter being operative to generate a beam of pulses ofelectromagnetic radiation,

a receiver connected to said electrical energy means, said receiverbeing responsive to electromagnetic radiation of the wavelengthgenerated by the transmitter and operative to generate correspondingpulses of electrical energy when the two objects are in a relativeposition such that the beam of pulses generated by the transmitterimpinges upon the receiver;

translating means operative from a first to a second condition whenenergized; and

. coupling means connected between the receiver and the translatingmeans for energizing the translating means only with the pulses ofelectrical energy generated by the receiver, said coupling meanscompletely isolating said translating means from said electrical energymeans, preventing operation of said translating means from saidelectrical energy means due to component failure.

2. The detector of claim 1 including a threshold device comprising:

a comparator having two inputs and an output; and

a reference signal source for generating a reference signal having apredetermined value;

said receiver being connected to one input of the comparator, thereference signal source being connected to the other input and thecoupling means being connected to the output,

said comparator being operative to deliver the pulses generated by thereceiver to the coupling means only when they reach a predeterminedvalue with respect to the reference signal.

3. Thedetector of claim 2 wherein said transmitter includes a functiongenerator operative to generate a repetitive electrical signal having afrequency in the kilohertz range and a light emitting diode energized bysaid function generator and operative to emit pulses of light at thefrequency of said repetitive electrical signal,

wherein said receiver includes a photoresponsive device responsive tothe pulses of light emitted by the light emitting diode and operative togenerate pulses of electrical energy of a corresponding frequency, and

wherein the coupling means is an isolation transformer having a primaryconnected to the output of said comparator and a secondary connected tosaid translating device and operative to couple only signals in thekilohertz range between the primary and the secondary.

4. A fail-safe position detector for determining when two objects are ina desired relative position with respect to each other comprising:

a transmitter operative to generate a beam of pulses of electromagneticradiation having a wavelength between approximately 2,000 and 10,000A,

said transmitter including a function generator operative to generate arepetitive electrical signal having a frequency in the kilohertz rangeand a light emitting diode energized by said function generator andoperative to emit pulses of light at the frequency of said repetitiveelectrical signal,

a receiver responsive to electromagnetic radiation of the wavelengthgenerated by the transmitter and operative to generate correspondingpulses of electrical energy when the two objects are in a relativeposition such that the beam of pulses generated by the transmitterimpinges upon the receiver,

said receiver including a photoresponsive device responsive to thepulses of light emitted by the light emitting diode and operative togenerate pulses of electrical energy of a corresponding frequency,

a translating device operative from a first to a second condition whenenergized,

an AC. coupling device connected between the receiver and thetranslating device for energizing the translating device withthe pulsesof electrical energy generated by the receiver,

said A.C. coupling device being an isolation transformer having aprimary connected to the output of said comparator and a secondaryconnected to said translating device and operative to couple onlysignals in the kilohertz range between the primary and the secondary,

and a threshold device including a comparator having two inputs and anoutput and a reference signal source for generating a reference signalhaving a predetermined value, said receiver being connected to one inputof the comparator, the reference signal source being connected to theother input and the AC. coupling device being connected to the output,said comparator being operative to deliver the pulses generated by thereceiver to the A.C. coupling device only when they reach apredetermined value with respect to the reference signal,

said comparator including an operational amplifier having two inputs andone output with said receiver connected to one input and said referencesignal source connected to the other, and

a Zener diode having a breakdown voltage of a predetermined thresholdvalue and an impedance device connected in series to the output of saidoperational amplifier, whereby the comparator will generate an outputonly when the pulses generated by the receiver have a value with respectto the reference signal which is determined by the breakdown voltage ofthe Zener diode and the transfer function of the operational amplifier.

5. The detector of claim 4 wherein the operational amplifier is anuncompensated microcircuit operational amplifier having a naturalfrequency in the megahertz range, whereby the oscillations due tofailure of the operational amplifier cannot be passed through theisolation transformer to energize the translating device.

6. The detector of claim 1 including said mounting means for mountingboth the transmitter and the receiver on a first one of the objects andguiding means for guiding the relative movement of the two objects suchthat the second object intersects the beam of pulses of theelectromagnetic radiation between the transmitter and receiver when thetwo objects are in the desired position.

7. The detector of claim 1 including the mounting means includes meansfor mounting both the transmitter and the receiver on a first one of theobjects and a control device mounted on the other object for controllingthe flow of said pulses of electromagnetic radiation between thetransmitter and the receiver.

8. The detector of claim 7 wherein the control device incorporates areflective surface effective to reflect the pulses of electromagneticradiation generated by the transmitter,

wherein said transmitter includes focusing means to direct said pulsesinto a beam along a line of sight, and

wherein said receiver includes focusing means making the receiverresponsive only to pulses striking it along a predetermined line ofsight,

said focusing means being oriented such that the beam of pulses from thetransmitter is reflected by the control device along the predeterminedline of sight of the receiver only when the two objects are in thedesired relative position with respect to each other.

9. A fail-safe position detector for determining when two objects are ina desired relative position with respect to each other comprising:

a transmitter operative to generate a beam of pulses of electromagneticradiation having a wavelength between approximately 2,000 and 10,000A,said transmitter including focusing means to direct said pulses into abeam along a line of sight,

a receiver responsive to electromagnetic radiation of the wavelengthgenerated by the transmitter and operative to generate correspondingpulses of electrical energy when the two objects are in a relativeposition such that the beam of pulses generated by the transmitterimpinges upon the receiver,

said receiver including focusing means making the receiver responsiveonly to pulses striking it along a predetermined light of sight,

a translating device operative from a first to a second condition whenenergized,

an A.C. coupling device connected between the receiver and thetranslating device for energizing the translating device with the pulsesof electrical energy generated by the receiver,

mounting means for mounting both the transmitter and the receiver ona,first one of the objects,

a control device mounted on the other object for controlling the flow ofsaid pulses of electromagnetic radiation between the transmitter and thereceiver,

said control device incorporating a reflective surface effective toreflect the pulses of electromagnetic radiation generated by thetransmitter,

said control device being a vane projecting perpendicular to thedirection of relative movement of the two objects with the plane of thereflective surface oriented parallel to said direction of relativemovement, and

wherein the mounting means includes means to mount the transmitter andreceiver on said first object such that their lines of sight intersectat a point in the plane of said reflective surface,

said focusing means of the transmitter and receiver being oriented suchthat the beam of pulses from the transmitter is reflected by the controldevice along the predetermined line of sight of the receiver only whenthe two objects are in the desired relative position with respect toeach other.

10. The detector of claim 9 including a non-reflective shield mounted onsaid first object blocking the line of sight of said receiver at a pointbeyond the point at which the reflective surface of said vane intersectssaid line of sight when said objects are in the desired position,whereby the receiver is protected from stray radiation.

11. In a transportation system including a structure having a pluralityof landings and a vehicle mounted for movement relative to the structureto serve the landings, a detector system for detecting the position ofthe vehicle relative to a selected landing and including:

a transmitter including a light emitting diode and a pulse generatoroperative to pulse said light emitting diode at a predeterminedfrequency,

a receiver including photosensitive means responsive to the pulses oflight generated by the transmitter and operative to generate electricalpulses at a corresponding frequency in response thereto,

mounting means for mounting the transmitter-and receiver such that thepulses of light generated by the transmitter impinge upon the receiverwhen the vehicle is in a desired position relative to the selectedlanding,

translating means operative from a first to a second condition whenenergized, and

an isolation transformer connected between the receiver and thetranslating means and operative to energize the translating means onlyin response to electrical pulses from the receiver.

12. In a transportation system including a structure having a pluralityof landings and a vehicle mounted for movement relative to the structureto serve the landings, a detector system for detecting the position ofthe vehicle relative to a selected landing and including:

a transmitter including an infrared light emitting diode and a pulsegenerator operative to pulse said infrared light emitting diode at afrequency in the kilohertz range,

a receiver including a phototransistor responsive to the pulses ofinfrared light generated by the transmitter and operative to generateelectrical pulses at a corresponding frequency in response thereto,

mounting means for mounting the transmitter and receiver such that thepulses of infrared light generated by the transmitter impinge upon thereceiver when the vehicle is in a desired position relative to theselected landing,

a translating device operative from a first to a second condition whenenergized, and

an isolation transfonner connected between the receiver and thetranslating device and operative to energize the translating device onlyin response to electrical pulses from the receiver having a frequency inthe kilohertz range,

and a vane having a metallic surface effective to reflect infraredlight, said mounting means including means for mounting said vane tosaid structure adjacent the selected landing and means for mounting thetransmitter and receiver on said vehicle such that the beam of pulses ofinfrared light transmitted by the receiver is reflected by the metallicsurface of said vane to impinge upon said receiver only when saidvehicle is in the desired position relative to the selected landing.

1. A fail-safe position detector for determining when two objects are ina desired relative position with respect to each other comprising:electrical energy means, a transmitter connected to said electricalenergy means, said transmitter being operative to generate a beam ofpulses of electromagnetic radiation, a receiver connected to saidelectrical energy means, said receiver being responsive toelectromagnetic radiation of the wavelength generated by the transmitterand operative to generate corresponding pulses of electrical energy whenthe two objects are in a relative position such that the beam of pulsesgenerated by the transmitter impinges upon the receiver; translatingmeans operative from a first to a second condition when energized; andcoupling means connected between the receiver and the translating meansfor energizing thE translating means only with the pulses of electricalenergy generated by the receiver, said coupling means completelyisolating said translating means from said electrical energy means,preventing operation of said translating means from said electricalenergy means due to component failure.
 2. The detector of claim 1including a threshold device comprising: a comparator having two inputsand an output; and a reference signal source for generating a referencesignal having a predetermined value; said receiver being connected toone input of the comparator, the reference signal source being connectedto the other input and the coupling means being connected to the output,said comparator being operative to deliver the pulses generated by thereceiver to the coupling means only when they reach a predeterminedvalue with respect to the reference signal.
 3. The detector of claim 2wherein said transmitter includes a function generator operative togenerate a repetitive electrical signal having a frequency in thekilohertz range and a light emitting diode energized by said functiongenerator and operative to emit pulses of light at the frequency of saidrepetitive electrical signal, wherein said receiver includes aphotoresponsive device responsive to the pulses of light emitted by thelight emitting diode and operative to generate pulses of electricalenergy of a corresponding frequency, and wherein the coupling means isan isolation transformer having a primary connected to the output ofsaid comparator and a secondary connected to said translating device andoperative to couple only signals in the kilohertz range between theprimary and the secondary.
 4. A fail-safe position detector fordetermining when two objects are in a desired relative position withrespect to each other comprising: a transmitter operative to generate abeam of pulses of electromagnetic radiation having a wavelength betweenapproximately 2,000 and 10,000A, said transmitter including a functiongenerator operative to generate a repetitive electrical signal having afrequency in the kilohertz range and a light emitting diode energized bysaid function generator and operative to emit pulses of light at thefrequency of said repetitive electrical signal, a receiver responsive toelectromagnetic radiation of the wavelength generated by the transmitterand operative to generate corresponding pulses of electrical energy whenthe two objects are in a relative position such that the beam of pulsesgenerated by the transmitter impinges upon the receiver, said receiverincluding a photoresponsive device responsive to the pulses of lightemitted by the light emitting diode and operative to generate pulses ofelectrical energy of a corresponding frequency, a translating deviceoperative from a first to a second condition when energized, an A.C.coupling device connected between the receiver and the translatingdevice for energizing the translating device with the pulses ofelectrical energy generated by the receiver, said A.C. coupling devicebeing an isolation transformer having a primary connected to the outputof said comparator and a secondary connected to said translating deviceand operative to couple only signals in the kilohertz range between theprimary and the secondary, and a threshold device including a comparatorhaving two inputs and an output and a reference signal source forgenerating a reference signal having a predetermined value, saidreceiver being connected to one input of the comparator, the referencesignal source being connected to the other input and the A.C. couplingdevice being connected to the output, said comparator being operative todeliver the pulses generated by the receiver to the A.C. coupling deviceonly when they reach a predetermined value with respect to the referencesignal, said comparator including an operational amplifier having twoinputs and one output with said receivEr connected to one input and saidreference signal source connected to the other, and a Zener diode havinga breakdown voltage of a predetermined threshold value and an impedancedevice connected in series to the output of said operational amplifier,whereby the comparator will generate an output only when the pulsesgenerated by the receiver have a value with respect to the referencesignal which is determined by the breakdown voltage of the Zener diodeand the transfer function of the operational amplifier.
 5. The detectorof claim 4 wherein the operational amplifier is an uncompensatedmicrocircuit operational amplifier having a natural frequency in themegahertz range, whereby the oscillations due to failure of theoperational amplifier cannot be passed through the isolation transformerto energize the translating device.
 6. The detector of claim 1 includingsaid mounting means for mounting both the transmitter and the receiveron a first one of the objects and guiding means for guiding the relativemovement of the two objects such that the second object intersects thebeam of pulses of the electromagnetic radiation between the transmitterand receiver when the two objects are in the desired position.
 7. Thedetector of claim 1 including the mounting means includes means formounting both the transmitter and the receiver on a first one of theobjects and a control device mounted on the other object for controllingthe flow of said pulses of electromagnetic radiation between thetransmitter and the receiver.
 8. The detector of claim 7 wherein thecontrol device incorporates a reflective surface effective to reflectthe pulses of electromagnetic radiation generated by the transmitter,wherein said transmitter includes focusing means to direct said pulsesinto a beam along a line of sight, and wherein said receiver includesfocusing means making the receiver responsive only to pulses striking italong a predetermined line of sight, said focusing means being orientedsuch that the beam of pulses from the transmitter is reflected by thecontrol device along the predetermined line of sight of the receiveronly when the two objects are in the desired relative position withrespect to each other.
 9. A fail-safe position detector for determiningwhen two objects are in a desired relative position with respect to eachother comprising: a transmitter operative to generate a beam of pulsesof electromagnetic radiation having a wavelength between approximately2,000 and 10,000A, said transmitter including focusing means to directsaid pulses into a beam along a line of sight, a receiver responsive toelectromagnetic radiation of the wavelength generated by the transmitterand operative to generate corresponding pulses of electrical energy whenthe two objects are in a relative position such that the beam of pulsesgenerated by the transmitter impinges upon the receiver, said receiverincluding focusing means making the receiver responsive only to pulsesstriking it along a predetermined light of sight, a translating deviceoperative from a first to a second condition when energized, an A.C.coupling device connected between the receiver and the translatingdevice for energizing the translating device with the pulses ofelectrical energy generated by the receiver, mounting means for mountingboth the transmitter and the receiver on a first one of the objects, acontrol device mounted on the other object for controlling the flow ofsaid pulses of electromagnetic radiation between the transmitter and thereceiver, said control device incorporating a reflective surfaceeffective to reflect the pulses of electromagnetic radiation generatedby the transmitter, said control device being a vane projectingperpendicular to the direction of relative movement of the two objectswith the plane of the reflective surface oriented parallel to saiddirection of relative movement, and wherein the mOunting means includesmeans to mount the transmitter and receiver on said first object suchthat their lines of sight intersect at a point in the plane of saidreflective surface, said focusing means of the transmitter and receiverbeing oriented such that the beam of pulses from the transmitter isreflected by the control device along the predetermined line of sight ofthe receiver only when the two objects are in the desired relativeposition with respect to each other.
 10. The detector of claim 9including a non-reflective shield mounted on said first object blockingthe line of sight of said receiver at a point beyond the point at whichthe reflective surface of said vane intersects said line of sight whensaid objects are in the desired position, whereby the receiver isprotected from stray radiation.
 11. In a transportation system includinga structure having a plurality of landings and a vehicle mounted formovement relative to the structure to serve the landings, a detectorsystem for detecting the position of the vehicle relative to a selectedlanding and including: a transmitter including a light emitting diodeand a pulse generator operative to pulse said light emitting diode at apredetermined frequency, a receiver including photosensitive meansresponsive to the pulses of light generated by the transmitter andoperative to generate electrical pulses at a corresponding frequency inresponse thereto, mounting means for mounting the transmitter andreceiver such that the pulses of light generated by the transmitterimpinge upon the receiver when the vehicle is in a desired positionrelative to the selected landing, translating means operative from afirst to a second condition when energized, and an isolation transformerconnected between the receiver and the translating means and operativeto energize the translating means only in response to electrical pulsesfrom the receiver.
 12. In a transportation system including a structurehaving a plurality of landings and a vehicle mounted for movementrelative to the structure to serve the landings, a detector system fordetecting the position of the vehicle relative to a selected landing andincluding: a transmitter including an infrared light emitting diode anda pulse generator operative to pulse said infrared light emitting diodeat a frequency in the kilohertz range, a receiver including aphototransistor responsive to the pulses of infrared light generated bythe transmitter and operative to generate electrical pulses at acorresponding frequency in response thereto, mounting means for mountingthe transmitter and receiver such that the pulses of infrared lightgenerated by the transmitter impinge upon the receiver when the vehicleis in a desired position relative to the selected landing, a translatingdevice operative from a first to a second condition when energized, andan isolation transformer connected between the receiver and thetranslating device and operative to energize the translating device onlyin response to electrical pulses from the receiver having a frequency inthe kilohertz range, and a vane having a metallic surface effective toreflect infrared light, said mounting means including means for mountingsaid vane to said structure adjacent the selected landing and means formounting the transmitter and receiver on said vehicle such that the beamof pulses of infrared light transmitted by the receiver is reflected bythe metallic surface of said vane to impinge upon said receiver onlywhen said vehicle is in the desired position relative to the selectedlanding.